The oft-repeated rationale is that this kind of bulk data collection has the power to prevent terrorist attacks and keep the US safe. But an analysis from the non-profit policy group New America Foundation, released today, shows that NSA surveillance programmes have been used to apprehend only 17 of the 225 people charged with an act of terrorism in the US since 9/11. The report also argues that the NSA's mass surveillance, which began after the Al-Qaida attacks on the World Trade Center and Pentagon, would not even have helped to prevent those attacks.

The key issue in the failure to prevent 9/11 was not a lack of information, the report claims, but poor sharing of existing intelligence between the CIA and other government departments.